In recent years, thermal transfer systems have been developed to obtain printed output from images generated from various electronic capturing devices and methods. An electronic image is first subjected to color separation by color filters. The respective color separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The donor element and dye receiving element are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta and yellow signals. The process is then repeated for the other two colors. The dye is transferred from these selectively heated regions of the dye donor sheet to the receiver sheet to form a pattern with a shape and intensity that corresponds to the pattern and intensity of the heat applied to the dye donor element. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271 by Brownstein entitled “Apparatus and Method for Controlling a Thermal Printer Apparatus,” issued Nov. 4, 1986, the disclosure of which is hereby incorporated by reference.
The thermal print-head contains a very hot print element that is in intimate contact with a dye donor ribbon. The current donor ribbon is designed to contain a heat resistant or slipping layer on one side to facilitate movement of the donor element across the print head. The opposite side is comprised of cyan, magenta and yellow dye and laminate that are patch coated on the opposite side of the support that is typically polyethylene terephthalate (PET).
The purpose of the slipping layer is to facilitate printing by providing a surface that can survive contact with the hot print head and is also lubricious enough to allow movement across the head.
The dye side of the patch coated donor ribbon contains alternating patches of cyan dye, magenta dye, yellow dye, and laminate that are printed in succession. Because few materials stick to PET, it is necessary to have an adhesive layer in between the slip and PET, as well as in between the PET and dye layers.
As the print is made, the receiver which is a dye receiving layer coated on paper support, moves through the printer, it receives the yellow dye, backs up, receives the magenta dye, backs up, receives the cyan dye, backs up, and lastly receives the protective laminate layer. The receiver is traditionally fed through the printer in sheet format. Print time for an 8×10 inch print is approximately 90 seconds with this method.
As the system evolves into higher volume applications, a multi-head printing system is necessary to allow faster printing times. When media was designed for the multihead printer, problems were found that were different from what were found with the cyan, magenta or laminate design. At the desired printing speed the thermal efficiency was not achieved with the yellow dye donor. Artifacts in the yellow dye donor were found, and higher printing energies were necessary to obtain the necessary maximum density to create acceptable photographic images.